1. Field of the Invention
The present invention relates generally to a novel drilling motor component. More particularly, the present invention relates to an improved stator and related methods of manufacture for a Moineau style motor.
2. Description of the Related Art
Referring to FIG. 1, in drilling a borehole 100 in the earth, such as for the recovery of oil, it is conventional practice to connect a drill bit 110 on the lower end of an assembly of drill pipe sections that are connected end-to-end so as to form a "drill string" 120. The drill string 120 is rotated and advanced downward, causing the drill bit to cut through the underground rock formation. A pump 130 on the surface 140 typically takes drilling fluid (also known as drilling mud), represented by arrows 135, from a mud pit 132 and forces it down through a passage in the center of the drill string 120. The drilling fluid then exits the drill bit 110, in the process cooling the face drill bit. The drilling mud returns to the surface 150 by an area located between the borehole and the drill string, carrying with it shavings and bits of rock from downhole.
A conventional motor (not shown) is typically located on the surface to rotate the drill string 120 and thus the drill bit. Often, a drill motor 160 that rotates the drill bit may also be placed as part of the drill string a short distance above the drill bit. This allows directional drilling downhole, and can simplify deep drilling. One such motor is called a "Moineau motor" and uses the pressure exerted on the drilling fluid 135 by the surface pump 140 as a source of energy to rotate the drill bit 110.
FIG. 2 is a cut-away top view of a prior art Moineau motor. Motor housing 210 contains an elastomeric rubber stator 220 with multiple helical lobes. The stator of FIG. 2 has 7 lobes, although a stator for a Moineau motor with as few as two lobes is possible. Three of these lobes are labeled 225. A typical stator lobe makes a complete spiral in 36 inches. This distance is known as the pitch length. Inside the stator 220 is a rotor 240, the rotor 240 by definition having one lobe fewer than does the stator. The rotor has an identical pitch length to that of the stator. The rotor 240 and stator 220 interengage at the helical lobes to form a plurality of sealing surfaces 260. Sealed chambers 250 between the rotor and stator are also formed. The rubber of the stator degenerates at areas 231-237 and at areas 271-277.
In operation, drilling fluid is pumped in the chambers 250 formed between the rotor and the stator, and causes the rotor to nutate or precess within the stator as a planetary gear would nutate within an internal ring gear. The centerline of the rotor travels in a circular path around the centerline of the stator. The gearing action of the stator lobes causes the rotor to rotate as it nutates. The nutation frequency is defined as the multiple of the number of rotor lobers times the rotor revolution speed. In the case of a six-lobed rotor, the centerline of the rotor travels in a complete circle six times for each full rotor rotation.
One drawback in such prior art motors is the stress and heat generated by the movement of the rotor within the stator. There are several mechanisms by which heat is generated. The first is the compression of the stator rubber by the rotor, known as interference. Interference is necessary to seal the chambers to prevent leakage and under typical conditions may be on the order of 0.005" to 0.030". The sliding or rubbing movement of the rotor combined with the forces of interference generates friction. In addition, with each cycle of compression and release of the rubber, heat is generated due to internal viscous friction among the rubber molecules. This phenomenon is known as hysteresis. Cyclic deformation of the rubber occurs due to three effects: interference, centrifugal force, and reactive forces from torque generation. The centrifugal force results from the mass of the rotor moving in the nutational path previously described. Reactive forces from torque generation are similar to those found in gears that are transmitting torque. In addition, heat may also be present from the high temperatures downhole.
Because elastomers are poor conductors of heat, the heat from these various sources builds up in the thick sections 231-237 of the stator lobes. In these areas the temperature rises higher than the temperature of the circulating fluid or the formation. This increased temperature causes rapid degradation of the elastomer. Also, the elevated temperature changes the mechanical properties of the rubber, weakening the stator lobe as a structural member and leading to cracking and tearing of sections 231-237, as well as portions 271-277 of the rubber at the lobe crests.
These forms of rubber degeneration are major drawbacks because when a downhole motor fails, not only must the motor be replaced, but the entire drillstring must be "tripped" or drawn from the borehole, section by section, and then re-inserted with a new motor. Because the operator of a drilling operation is often paying daily rental fees for his equipment, this lost time can be very expensive, especially after the substantial cost of an additional motor.
One known approach to increase the durability of a Moineau motor is to reduce the interference of the motor so that less heat is generated. However, this will reduce the torque available to rotate the downhole drill bit and so may not be an acceptable alternative. Another solution to the durability problem may be to lengthen the motor so that less heat is generated per foot of motor length. However, this approach imposes additional cost and weight to the motor. Further, depending upon the application downhole, a longer motor may not be desirable.
Other configurations for Moineau motors have also been suggested, such as U.S. Pat. No. 4,676,725 to Eppink and U.S. Pat. No. 5,171,138 to Forrest. However, many of these configurations are undesirably complex from a manufacturing perspective, and thus can be very expensive to make. In addition, some of these concepts limit the cross-sectional area or do not provide good paths for heat conduction.
Other problems have also existed in the prior art motors, and thus a downhole motor is needed that solves or minimizes many of these problems. Ideally, such an improved motor would provide improved structural integrity and heat conduction, thereby leading to increased durability and reduced failure from degeneration of the elastomeric portions of the rotor and stator downhole. Alternately, such an improved motor could be shorter or have greater power than a prior art motor, while maintaining good durability. Further, such a motor should solve other problems present in the prior art and should be manufacturable at a low cost so that it can attain widespread use by the industry.